Answer by diggy for Load WordPress RSS Feed's in a custom PHP file

do_feed() needs $wp_query and a feed query var to be set. So I guess you could do a custom query and set the feed var:

if( empty( $doing_rss ) ) {
    $doing_rss = 1;
    require(ABSPATH . '/wp-blog-header.php');
}

$query = new WP_Query( array( 'post_type' => 'post' ) );

global $wp_query;

$wp_query = $query;

$wp_query->query_vars['feed'] == 'rss2';

do_feed();